$ \left(\dfrac{25}{16}\right)^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{16}{25}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{16}{25}$ Figure out what goes in the blank: $\Big({\dfrac{4}{5}}\Big)^{2}=\dfrac{16}{25}$ So $\left(\dfrac{25}{16}\right)^{-\frac{1}{2}}=\left(\dfrac{16}{25}\right)^{\frac{1}{2}}=\dfrac{4}{5}$